1. Field of the Invention
The present invention generally relates to welding equipment and processes. More particularly, this invention relates to a welding apparatus adapted to deposit an overlay weld on a rotating tube, wherein the quality of the weld and the operation of the apparatus are enhanced as a result of structural and control features of the apparatus.
2. Description of the Related Art
Overlay welding generally involves depositing weld material over a surface region in a series of beads with some lateral overlapping, forming a continuous layer of weld material that increases the thickness and strength of the region. Overlay welds are often deposited by shielded metal-arc welding in the restoration of tubes and vessels used in industries such as utilities, co-generation refining, petrochemical, pulp and paper, and waste-to-energy. A particular use of overlay welds is in the reinforcement and repair of tubular members, such as boiler tubes, small pressure vessels, nozzles and pipe sections of a boiler. Boiler tubes provided with an overlay weld coating offer much longer tube life, reducing costly unscheduled maintenance outages and significantly improving boiler availability.
Overlay weld material can be deposited on tube stock manually or with the use of an automatic or semiautomatic welding machine on which the tube stock is mounted. An example of an automatic overlay welding apparatus is disclosed in U.S. Pat. No. 6,013,890 to Hulsizer. The apparatus is configured to horizontally support and rotate a tube stock on its axis as the overlay weld is deposited, resulting in what can be termed a spiral weld. An overlay weld is deposited by a first welding head (preferably a metal inert gas (MIG) torch), and then washed with a second welding head (preferably a tungsten inert gas (TIC) torch) to eliminate a heat affected zone (HAZ) formed in the surface of the tube stock when the overlay weld was deposited. Water is flowed through the tube stock during the welding operation in order to control the temperature of the tube sock and the rate of cooling of the overlay weld. The Hulsizer apparatus makes use of a wheeled carriage that carries the overlay and wash welding heads, and travels along the length of a frame that supports the tube stock being welded. Tube stock supports are placed along the length of the frame to support the rotating tube, and must be moved occasionally to avoid the wheeled carriage as it travels the frame length.
Accuracy and consistency of all parameters are critical during an overlay welding process, especially when the article being welded must be both supported and manipulated during welding, as is the case with spiral overlay welding operations performed on rotating tube stock. Further complicating the overlay process is the need to comply with a wide range of specifications, such as minimal weld penetration, low dilution, complete fusion, homogeneous deposits, very low heat input, and minimum deposit thickness, e.g., on the order of 0.060 to 0.070 inch (about 1.5 to 1.8 mm). A variety of welding defects can occur in an overlay weld process, including cold laps on weld overlay starts and stops, burn-throughs, and overlay deposited with improper shielding gas pressure (necessitating removal and re-application of the weld). Furthermore, with existing overlay welding equipment, tube stock is prone to overheating and deformation as a result of being inadequately cooled and supported. Existing overlay welding equipment also have a limited level of integration in terms of controlling various critical welding parameters, such as shielding gas pressure, wire feed rates, tube rotation speed, carriage travel speed, etc. Any of these parameters can, if not properly controlled, lead to inconsistencies in the thickness of the weld deposit, which may necessitate the removal and re-application of the overlay.
In view of the above, it would be desirable if an improved overlay welding apparatus were available for depositing overlay welds on tube stock.
The present invention provides an overlay welding apparatus for depositing an overlay weld on rotating tube stock, and which provides improved support and thermal management for the tube stock and improved control of the welding parameters, with the overall effect of increasing the quality and consistency of the overlay weld. More particularly, the invention is an electric-arc welding overlay apparatus with improvements that render the apparatus more reliable and highly integrated in comparison to prior art apparatuses used to deposit overlay welds on tube stock.
The apparatus of this invention generally comprises an elongate frame and first and second support means for supporting a tube stock relative to the frame and for rotating the tube stock about a central axis thereof. The tube stock is inclined relative to horizontal when supported by the first and second support means, such that improved cooling is achieved with a cooling medium delivered to the interior of the tube stock as the tube stock rotates on the frame. The apparatus further comprises a welding carriage mounted to the frame and adapted for travel along the frame. The welding carriage supports an overlay welding head for depositing an overlay weld on a surface region of the tube stock as the tube stock is rotated, and further supports a wash welding head for reducing a heat-affected zone in the surface region as the tube stock is rotated. A steady rest is mounted adjacent the welding carriage for movement with the welding carriage along the frame. The steady rest is adapted to rotatably support a portion of the tube stock in close proximity to the surface regions of the tube stock on which the overlay weld is being deposited and the heat-affected zone is being reduced by the overlay and wash welding heads, respectively, so as to better support the tube stock and reduce deformation of the tube stock during the welding operation.
The welding apparatus also preferably includes electronic circuitry that provides better control of the overlay and wash welding heads along the desired weld path, as well as better control of tube stock rotation, welding current, wire feed speed and oscillation speed. The apparatus also preferably includes a feedback capability that can address errors and failures of the various components of the apparatus, including shielding gas, tube stock rotation, carriage travel, etc., thereby reducing the incidence of cold laps, burn-throughs, and overlay deposited with improper shielding gas pressure. The invention utilizes the above improvements and preferred aspects to provide an overlay welding apparatus that is highly integrated and reliable as compared to overlay welding apparatuses of the prior art, yet is sufficiently uncomplicated to be used by operators with limited experience.
Other objects and advantages of this invention will be better appreciated from the following detailed description.